Complex microelectronic devices such as modern semiconductor chips require numerous connections to other electronic components. For example, a complex microprocessor chip may require hundreds of connections to external devices.
Semiconductor chips have commonly been connected to electrical traces on mounting substrates using several alternative methods, including wire bonding, tape automated bonding and flip-chip bonding. Each of these techniques presents various problems including difficulty in testing the chip after bonding, long lead lengths, large areas occupied by the chip on the microelectronic assembly, and fatigue of the connections due to changes in the sizes of the chip and the substrate under thermal expansion and contraction.
Structures that have been used to successfully address the foregoing problems are disclosed in commonly assigned U.S. Pat. Nos. 5,148,265; 5,148,266; and 5,455,390. Structures according to the embodiments taught in these patents comprise a flexible, sheet-like element having a plurality of terminals disposed thereon. Flexible leads are used to connect the terminals with contacts on a first microelectronic element such as an integrated circuit. The terminals may then be used to test the microelectronic chip, and may be subsequently bonded to a second microelectronic element. The flexible leads permit thermal expansion of various components without inducing stresses in the connection.
Commonly assigned U.S. Pat. No. 5,518,964 ("the '964 patent"), hereby incorporated in its entirety herein, discloses further improvements in microelectronic connections. In certain embodiments of the '964 patent, a flexible, sheet-like element has a first surface with a plurality of elongated, flexible leads extending from a terminal end attached to the sheet-like element to a tip end offset from the terminal end in a preselected, first horizontal direction parallel to the sheet-like element. The tip ends have bond pads for connection to a microelectronic element. As the term is used herein, "microelectronic element" encompasses circuit boards, integrated circuits, connection components such as polyimide or other dielectric sheets, and other components used in microelectronic circuitry. Each of the plurality of leads is simultaneously formed by moving all of the tip ends of the leads relative to the terminal ends thereof so as to bend the tip ends away from the sheet-like element. This is accomplished by relative movement between the sheet-like element and the microelectronic element.
The tip ends of the leads are initially attached to the sheet-like element. The initial position of the bond pad on the tip ends is thereby fixed with respect to the terminal ends in order to facilitate attachment to the microelectronic element.
Various lead configurations are disclosed in the '964 patent. In one such configuration, the leads comprise straight, elongated bodies of conductive material extending between terminal ends connected to a dielectric sheet-like element and tip ends to be connected to a microelectronic element. The terminal end of the lead is attached through a via in the sheet-like element to another microelectronic element on the other side of the sheet-like element.
The attachment of the tip ends of the leads to the sheet-like element is releasable. After bonding the tip ends to the microelectronic element, the leads are formed in their final configuration by moving the sheet-like element and the microelectronic elements relative to each other in two directions: in a vertical direction away from each other, and in a horizontal direction parallel to the sheet-like element. As a result, the tip end of the lead is separated from the sheet-like element and traces an arcuate path relative to the other end of the lead. That movement prevents stretching of the lead during formation and results in an S-shaped configuration of the lead that is advantageous in absorbing further relative movement between the sheet-like element and the microelectronic element due to thermal expansion/contraction during operation of the resulting device.
In another lead configuration taught in the '964 patent, the lead is initially a curved strip disposed on a surface of the sheet-like element. A terminal end of the lead is connected to a terminal through a via in the sheet-like element and a tip end is bonded to a microelectronic element. In forming those leads to a final configuration, the sheet-like element and the microelectronic element are moved away from each other in a vertical direction only. The curve of the lead is partially straightened by the relative movement of the elements. The "slack" created by the initial curve in the lead permits vertical displacement of the microelectronic components without the necessity of providing additional lead length by horizontally displacing the components.
A number of such configurations of curved leads are disclosed in the '964 patent. An S-shaped lead structure permits nesting of adjacent leads in configurations requiring a high lead density. A U-shaped lead configuration permits a larger relative displacement of the microelectronic components in a vertical direction without a corresponding horizontal displacement. Numerous other lead configurations are contemplated in the '964 patent.
Still further improvement in the above-described configurations and processes would be desirable.